In recent years, particulates, such as soot, contained in exhaust gases that are discharged from internal combustion engines of vehicles, such as buses and trucks, and construction machines, have raised serious problems as those particulates are harmful to the environment and the human body.
Various honeycomb structural bodies made of porous ceramics, which serve as filters capable of collecting particulates in exhaust gases to purify the exhaust gases, have been proposed.
Conventionally, with respect to the honeycomb structural body of this type, a filter having the following structure has been proposed in which: two kinds of through holes, that is, a group of through holes with a relatively large capacity (hereinafter, may be referred to as large-capacity through hole) and a group of through holes with a relatively small capacity (hereinafter, may be referred to as small-capacity through hole), are prepared, and the end on the exhaust gas outlet side of each through hole belonging to the group of large-capacity through holes is sealed with a plug, with the end on the exhaust gas inlet side of each through hole belonging to the group of small-capacity through holes being sealed with a plug, so that the surface area of the through holes with the opened inlet side (hereinafter, may be referred to as inlet-side through holes) is made relatively greater than the surface area of the through holes with the opened outlet side (hereinafter, may be referred to as outlet-side through holes); thus, it becomes possible to suppress an increase in pressure loss upon collecting particulates (for example, see Patent Document 1, Patent Document 2 and Patent Document 3). With respect to the filter of this type, filters in which the shapes of the inlet-side through holes and the outlet-side through holes are respectively formed into an octagonal shape and a quadrangular shape have been proposed (for example, see Patent Document 4 and Patent Document 5).
Moreover, another filter has been proposed in which: the number of the inlet-side through holes is made greater than the number of the outlet-side through holes so that the surface area of the group of inlet-side through holes is made relatively greater than the surface area of the group of outlet-side through holes (for example, see Patent Document 6 and FIG. 3 of Patent Document 7).
In the case of the honeycomb structural bodies to be used as the exhaust gas purifying filters disclosed in Patent Documents 1 to 7, in comparison with a honeycomb structural body in which the total amount of the surface area of the group of the inlet-side through holes and the total amount of the surface area of the group of the outlet-side through holes are the same, since the total amount of the surface area of the inlet-side through holes is made relatively greater; thus, the deposition layer of collected particulates becomes thinner, thereby making it possible to suppress an increase in a pressure loss at the time of collecting particulates, and also to increase the collection limit amount of particulates.
Moreover, after having collected a predetermined amount of particulates, an engine controlling process is carried out to raise the exhaust gas temperature or the temperature of a heater placed on the upstream side of exhaust gases from the honeycomb structural body so that a regenerating process that makes the particulates in contact with high-temperature gases to burn the particulates is carried out; and in this case, the burning rate of the particulates can be increased by making the thickness of the deposition layer of the particulates thinner.
Among the honeycomb structural bodies in which the surface area of the group of inlet-side through holes is made relatively greater than the surface area of the group of outlet-side through holes, in the case of the honeycomb structural body having a greater aperture rate, a low density and a low thermal capacity can be achieved because the rate of the capacity of the through holes that occupy the honeycomb structural body increases; therefore, it becomes possible to provide a quick temperature-raising process, and consequently to improve the response property exerted upon heating exhaust gases.
However, the filters having a greater aperture rate on the inlet side have a problem that cracks tend to occur in the plugs that seal the group of inlet-side through holes and the vicinity thereof (hereinafter, may be referred to as outlet-side sealing portions) at the time of the regenerating process.
Conventionally, with respect to the thickness of the plugs in the honeycomb structural body, for example, Patent Document 8 describes that the thickness is desirably set in a range from 2 to 5 mm.
Patent Document 1: JP-A 56-124417 (1981)
Patent Document 2: JP-A 62-96717 (1987)
Patent Document 3: U.S. Pat. No. 4,364,761
Patent Document 4: WO 02/10562A1 pamphlet
Patent Document 5: FR Patent No. 2789327
Patent Document 6: JP-A 58-196820 (1983)
Patent Document 7: U.S. Pat. No. 4,417,908
Patent Document 8: US Patent No. 2003/0041730A1